1. Field of the Invention
This invention relates to an optical pickup device for recording and reproducing data in a video disk player, compact disk player or the like.
2. Background of the Invention
An optical pickup device is known in which focus control is carried out using astigmatism and in which an optical detector is used which is divided into three parts by two substantially parallel lines. Such disclosure appears, for instance, in Japanese Patent Application (OPI) Nos. 87328/1980 and 45947/1985.
FIG. 1 shows the principle of the conventional optical pickup device. In FIG. 1, a laser beam emitted from a light source 1, such as a semiconductor laser is applied to a beam splitter (half-prism) 2. The light beam reflected from the beam splitter 2 is focused on a rotating disk 4 with an objective lens 3. The light beam reflected from the disk 4 is applied through the objective lens 3 and the splitter 2 to a cylindrical lens 5, where it is given astigmatism. The light beam thus treated is detected by an optical detector 6.
As shown in FIG. 2, the optical detector 6 is divided by two substantially parallel lines into three regions A, B and C in such a manner that region B is located between regions A and C. As was described above, the light beam has been given astigmatism. Therefore, when the light beam is focused on the disk 4, a substantially circular light spot is formed on the optical detector 6. As the disk 4 is moved away from the focused position, a substantially elliptical light spot which is horizontally or vertically elongated is formed. The sum of the outputs of regions A and C is calculated by an addition circuit 8. The difference between the sum of the A and C outputs and the output of region B is obtained by a differential amplifier 7. The width of region B is selected so that the output of the differential amplifier 7 is zero when the disk 4 is at the focused position. Accordingly, as the disk 4 is moved away from the focalization position, the differential amplifier 7 provides an output corresponding to the movement. Therefore, the output can be employed as a focus error signal which controls the position of the objective lens 3. (That is, focus control is accomplished.)
When the focus error signal does not become zero although the disk is at the focused position, offset adjustment is carried out by moving the optical detector 6 along the optical axis when the two division lines are in parallel or by moving it in a plane perpendicular to the optical axis when the two division lines are not in parallel. The tracking direction is substantially in parallel with the two division lines. Therefore, the focus error signal is free from the movement of the light spot which is carried out for tracking control.
In the case where a cylindrical lens is employed as optical means for providing astigmatism, other aberrations will scarcely occur. On the other hand, in the case where a parallel planar plate is employed, not only astigmatism but also other aberrations take place so that the light spot is not circular when the focalization is obtained. As a result, when the light beam goes across the tracks on the disk, the components of diffracted light of the tracks, i.e., so-called "push-pull signals" are not canceled out between the regions. Thus, the resultant focus error signal is erroneous and focus servo control cannot accurately be achieved.